The national security of the United States of America (USA), along with many other countries around the globe, is at risk of attack by nuclear and/or radioactive weapons. The USA and international community need detectors to expose these threats at the borders of the nations, airports, and sea ports. The resolution for semiconductor-based gamma radiation detectors is defined as the full width at half max of a peak (FWHM) divided by the energy of the peak. The ideal characteristic for this resolution would be an impulse function. This, however, is not typically the case and the signals can be hard to resolve. For semiconductor-based gamma radiation detectors it is the noise within the detector that is responsible for some of the change from the ideal scenario. Currently, high purity germanium (HPGe) detectors offer possibly the best performance for detecting gamma photons and yield a resolution of about 0.2%. However, because of a narrow energy bandgap (Eg=0.7 eV), HPGe detectors are operated at cryogenic temperatures to operate properly, typically below 110K. This low bandgap value allows a relatively large amount of thermally generated current which degrades the signal to noise ratio in the detector, thus prompting the low temperatures of operation. The cooling requirement of Ge is an encumbrance and a room temperature detector would be greatly preferred, to allow for greater portability, operating efficiency, and ease of use.
At present, the only commercially available room-temperature (Eg=1.6 eV) alternative to cryogenically-cooled germanium detectors is based on Cadmium Zinc Telluride (CdZnTe or CZT), which has a resolution of about 10 times greater than Ge based gamma detectors. High resolution gamma detectors may be used for unambiguous identification of special nuclear materials. FIG. 1 shows an energy spectrum using three different detector materials, Germanium (Ge), Cadmium Zinc Telluride (CZT), and Sodium Iodide (NaI) exposed to 662 keV gamma energy. Only the Ge material is able to resolve the Special Nuclear Material (SNM) signatures with high enough accuracy, as is evidenced by the narrow and tall peaks. NaI resolution is very poor, as can be seen by the lack of distinct peaks. CZT peaks are evident, but to obtain the resolution desired for use in discovering unambiguously special nuclear materials, the peaks should be taller and the width of the peak should be narrower. This sort of peak probably cannot be realized with CZT unless the dark current in the sensor is minimized somehow. Dark current, or leakage current, in a gamma detector is the amount of electrons and/or holes that enter the semiconductor material used to detect gamma radiation from electrodes thereof and travel across the semiconductor material or the amount of electrons and holes thermally generated in the bulk material. Dark current decreases the performance of any gamma detector material. However, improvement in the CdZnTe may result in a detector material that can resolve the SNM signatures in a similar capacity as that of Ge.
Therefore, a CdZnTe-based gamma radiation detector that can operate effectively at or near room temperatures and still provide suitable resolution would be very beneficial.